Circuits of data or signal processing contain memories to store data. To fulfill high-speed processing requirements of circuits, it would be necessary to minimize both read and write time of memories. An example of such memories is SRAM (Static Random Access Memory) with capability to provide rapid access of contents stored therein. Nevertheless, SRAM retains its contents as long as electrical power is applied to the circuits. If the power is turned off or lost temporarily, its contents will be lost. Moreover, SRAM, which generally has a 6-transistor memory cell structure, may inevitably consume a large space of circuits.
The circuits may need another type of memories to retain data while power is turned off. Flash memory has an advantage in that data recorded therein is still preserved even power is turned off. However, speed of writing data into flash memory is relatively slower as compared to that of SRAM.